Conventional Lead-Acid Batteries
Conventional lead-acid batteries generally comprise a series of separate (monopolar) positive and negative electrodes, connected in a combined series and parallel arrangement to achieve the desired voltage and current. Each electrode usually consists of a grid constructed of lead (Pb), or a lead alloy which is filled with and covered with an active electrode material. Lead dioxide is used as the active electrical material for the positive electrode, and sponge lead is used for the negative electrode in a fully charged battery. The purpose of the grid is twofold: to contain the active material so that the electrodes may be suspended in the sulfuric acid electrolyte solution, and to collect and to conduct the electrical current generated by the active materials, so it can be transferred to the outside of the battery. The grid is ordinarily constructed of lead metal for four reasons:
1. Lead is electrically conductive; PA1 2. Lead is resistant to corrosion in the sulfuric-acid electrolyte solution; PA1 3. The metal is relatively inexpensive, as compared to more resistant but unaffordable materials such as gold or platinum; and PA1 4. Lead has exceptionally high gassing over-voltages for both hydrogen and oxygen, which minimizes the electrolytic decomposition of water in the electrolyte, and maximizes the formation efficiency of the active electrode materials, lead and lead dioxide. PA1 1. The excessive weight required to achieve high-power discharges needed by electric vehicles; PA1 2. The excessive weight required to store appreciable energy; and PA1 3. The batteries have insufficient life to be economical.
The construction of conventional lead-acid batteries results in a number of problems which limit their usefulness in two important applications: electric automobiles and electric utility energy storage. The three main problems of prior lead-acid batteries are:
The excessive weight of lead-acid batteries is due to the extensive use of lead, one of the heaviest natural materials. The power of lead-acid batteries is largely limited by the use of a grid design to collect and conduct the electrical current, which forces the current to travel along a high resistance path that limits the useful power from the battery.
Conventional versus Bipolar Lead-Acid Batteries
FIG. 1 depicts a conventional lead-acid battery. An external case CS and internal partitions PAR enclose cells which contain positive (+) and negative (-) plates deployed in a spatially parallel arrangement. These plates are grids that are characterized by a pattern of indentations or open spaces which are covered with an active material. The positive plates are covered with a positive active material PAM, and the negative plates are covered with a negative active material NAM. FIG. 1 shows a pair of positive (+) and negative (-) grids in cross-section GCS bearing these active materials. Positive and negative grids are segregated within the partitions PAR by separators SEP. The spaces around the plates that come into contact with the positive and negative active materials are filled with a sulfuric acid electrolyte EL. The plates are connected to a pair of terminals T that reside on the outside of the case CS.
The 6-Volt bipolar lead-acid battery shown in FIG. 2 is fundamentally different from the conventional 6-Volt lead-acid battery portrayed in FIG. 1. A case CS having protruding terminals T encloses a group of plates that are arranged in a spatially parallel configuration, but the plates in the bipolar battery and the way they are connected are quite dissimilar from the conventional battery. Each electrode in a bipolar battery comprises a separate grid containing either the positive or negative materials, and is suspended in a battery cell. Bipolar battery construction utilizes a series of bipolar battery plates called "biplates" BP. These biplates BP are solid sheets of material that divide the battery into cells and provide electrical contact between the positive and negative electrode materials of adjacent cells. A positive grid PG and negative grid NG are compared to a biplate BP in FIGS. 3 and 4.
The stack of biplates BP shown in FIG. 2 is held together by endplates EP. The positive side of each biplate BP is covered by a positive active material PAM, while the negative side of each biplate is covered by a negative active material NAM. The spaces between the biplates BP contain separators SEP and electrolyte EL. The areas ESA around the lateral surfaces of each hiplate BP may be fitted with some type of edge seal. In the bipolar battery, the electrical current needs to pass only through the thin bipolar plates BP, which also serve as the physical partitions between the cells. The electric current can, therefore, pass through the entire battery in a direction perpendicular to the plane of each biplate BP. This arrangement presents a very large cross sectional area and very short distance for the current to pass between cells, compared to the small electrical cross section of the grid and long electrical path to the next cell which is encountered in ordinary batteries. As a consequence of these large geometrical differences between the electrical paths in conventional versus bipolar batteries, the electrical resistance in the bipolar battery is approximately one fifth of conventional batteries. With this reduction in internal resistance, a high power battery suitable for electrical automobile propulsion becomes possible, provided the battery does not have excessive weight, can be constructed at an affordable cost, and also has a sufficiently long life.
The life of previous conventional and bipolar lead-acid batteries is limited by a number of failure modes. The two most common of these are the microscopic morphological degradation of the positive active material (PbO.sub.2) and the corrosion of the lead material used to construct the positive grids. The effects of microscopic morphological degradation have been largely overcome by the use of electrically conductive glass fibers that are coated with doped stannic oxide and placed within the active material, as described in my U.S. Pat. No. 4,507,372. The corrosion of lead, however, cannot be completely overcome because of its intrinsic thermodynamic instability at the electrical potential, i.e., approximately 1.75 volts relative to hydrogen (H.sub.2 /H.sup.+) found at the positive electrode in lead-acid batteries. This results in the slow corrosion of the lead and the formation of lead dioxide, which is mechanically weak. Eventually, sufficient corrosion of the lead grids in the positive electrode occurs and the battery performance degrades to a useless level. In contrast, the negative electrode grid suffers from no such corrosion because lead is thermodynamically stable at the negative electrode potential, 0.36 volts below the hydrogen electrode (H.sub.2 /H+). The negative lead grid, therefore, has an indefinite life. The foregoing discussion, although referring to the lead grids used in today's monopolar batteries, is relevant to the present invention because the same operating environments are present, i.e., sulfuric acid electrolyte and two different electrical potentials.
Unlike grids, however, the bipolar plate must simultaneously withstand a pair of positive and negative electrochemical reactions, oxidation and reduction. As a consequence, the first plates used to construct bipolar lead-acid batteries were made of solid lead, like their grid counterparts in conventional batteries. These bipolar plates were impractical for most applications, however, because of their heavy weight and the relentless corrosion. Eventually this corrosion results in a perforation of the biplate. The perforation causes an immediate electrical short between the cells, destroying cell integrity and degrading the battery. Several attempts to construct a practical plate for bipolar batteries are illustrated in FIGS. 5, 6, 7, 8, 9 and 10. One of the greatest challenges confronting developers of the bipolar lead-acid battery has been the construction of a bipolar plate which is light-weight, but which does not achieve the reduced weight by adding more cost or by compromising power capacity or useful lifetime. Each of the six different types of previous biplates, shown in FIGS. 5 through 10, are beset by their own particular shortcomings. The first lead battery plates, pictured in FIG. 5, were soft and difficult to work with. Repeated charging and discharging first creates corrosion on the plate surface. This corrosion creates areas of high electrical resistance. Eventually, the plate becomes perforated and the battery fails. Several years ago, a carbon-in-plastic plate C/P, like the one shown in FIG. 6, was developed. This hybrid plate fails quickly because the carbon oxidizes and forms acetic acid and carbon dioxide. The carbon-in-plastic plate was improved by incorporating solid lead spheres in the plastic plate, as shown in FIG. 7. This invention is described in my U.S. Pat. No. 4,658,499. Later, the carbon-in-plastic plate was improved somewhat further by adding a second layer of plastic containing the same conductive glass fibers as used in the positive electrode material. This improvement is described in my U.S. Pat. No. 4,507,372. The resulting combination plate is depicted in FIG. 8. A more complex design, which is portrayed in FIG. 9, adds a third layer of pure lead to the double plastic plate. Yet another attempt at providing a biplate for a high-power, bipolar battery is revealed by FIG. 10. This apparatus, called a "quasi-bipolar plate", includes a wrapping of lead that envelopes a plastic center, in which conduction is not through the plate as with a true biplate, but occurs around the folded edge. Although lighter in weight than pure lead, none of these hybrid or composite biplates has proven to be as good an electrical conductor or as corrosion-resistant and reliable, or as inexpensive, as the original lead plate.
Two previous bipolar battery designs are revealed in FIGS. 11 and 12. FIG. 11 is an illustration of a Sealed Bipolar Multi-Cell Battery, which is described in my U.S. Pat. No. 4,539,268. This low maintenance battery LMB has a pair of terminals T and a resealable vent V protruding from a housing H. The housing H encloses positive and negative plates PP and NP that are separated by fiberglass mats M. FIG. 12 provides an illustration of the stack configuration of the Lightweight Bipolar Storage Battery, which is described in my U.S. patent application Ser. No. 07/516,439 filed on Apr. 30, 1990. The exploded view in FIG. 12 shows a bipolar plate battery B which includes a top cover TC that protects an end plate EP, a current collector plate CC, and a current removing element CR. The enclosure E at the bottom of the assembly includes two leads L protruding from it and is designed to hold a similar group of elements that includes another end plate EP, current collector plate CC, and current removing element CR. All of these elements surround a central stack S of bipolar plates.
Despite all of these proposed solutions, the central goal of providing a biplate so that a powerful, lightweight, and practical bipolar lead-acid battery can be manufactured has remained elusive. The previous biplates are either expensive to manufacture, are extremely heavy, or are susceptible to debilitating corrosion because the plates oxidize rapidly under the severe acidic and electrical environments within the battery. The practical bipolar plate must possess a combination of critical characteristics: sufficient electrical conductivity, resistance to the different corrosion mechanisms occurring simultaneously on both the positive and negative sides of the plate, low weight, and low cost. The problem of developing a biplate with these characteristics for the bipolar lead-acid battery has presented the major obstacle to its successful development and commercialization. The achievement of a light yet powerful lead-acid storage battery that would be suitable for applications such as electric automobiles would constitute a major technological advance useful in both the automotive industry and the electrical power storage business. The enhanced performance that could be achieved using such an innovative device would satisfy a long felt need within the power and transportation industries.